Revert "Run tests also in Windows and macOS in GH Actions"

This reverts commit 2320c47.

Author: bonigarcia

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
       matrix:
         project: [ junit4, junit5, junit5-seljup, testng ]
         build-tool: [ mvn, gradle ]
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-latest ]
         java: [ 11 ]
     runs-on: ${{ matrix.os }}
     env: